The invention relates to a method and an apparatus for producing biogas from organic substances, substrate being supplied to a container by a feed system and there being arranged in the container at least one agitator mechanism.
The method serves for generating biogas from organic substances. The raw substance used in such plants for generating biogas is designated as substrate. The substrate is composed of biologically degradable biomass, such as liquid manure, silage or biowaste. The containers used for the production of biogas are also designated as bioreactors or fermenters. When the biogas plants are being operated continuously, substrate is supplied continuously to the container and biogas and also fermentation residue are extracted. Substrate located in the container is converted by means of various types of microorganisms. This biomass to be converted is designated as fermentation substrate. Microbial breakdown gives rise from the fermentation substrate to methane and carbon dioxide as the main components of the biogas.
The substrate supplied is mixed with the container contents. The substrate is supplied mostly by punctiform feed with the aid of fodder systems. The biomass dwell time required for as high a biogas yield as possible is dependent critically upon the mixing of the substrate with the fermentation substrate. In the case of those media which are characterized predominantly by increased viscosities, circulation of the container contents is necessary for mixing and/or intermixing, this taking place, as a rule, by means of agitator mechanisms.
In the sector of anaerobic bioreaction technology, fermenters with a height to diameter ratio greater than 0.5 are used in many applications. Mixing is in this case carried out mostly by means of vertical agitator mechanisms. The agitator mechanism propellers are in this case located on a central shaft driven from outside. The drive shaft is arranged vertically and projects into the container from above, while it mostly runs parallel to the container walls. Such a fermenter is known, for example, from German patent publication no. DE 199 47 339 A1.
Instead, the method according to the invention for producing biogas employs container forms, of which the height to diameter ratio is lower than 0.5. The diameter of the containers preferably lies between 16 and 40 meters. With these container dimensions, it is no longer economically viable to use a central vertical agitator mechanism driven from outside. For mixing the container contents, use is made of agitator mechanisms which are arranged predominantly in the marginal zone of the container and in the latter and which generate mostly horizontal flow of the medium in the container. Such an arrangement is known, for example, from US patent publication No. US 1012/0009664 (=WO 2008/104320).
German utility model No. DE 20 2007 002 835 U1 discloses a plurality of agitator mechanisms for intermixing the container contents, two agitator mechanisms arranged one above the other being arranged opposite an individual agitator mechanism. For high fermentation process efficiency, as uniform a biomass distribution as possible in the fermenter liquid is considered necessary. In addition, a filling level measurement device is provided, by means of which the filling height in the container is detected and a corresponding filling level measurement signal is generated as a height actual-value signal. A filling level measurement signal is delivered to a control device which, when a lower filling height is detected, activates a height servomotor for the agitator mechanism designed as a submersible motor agitator, such that the latter is lowered and its agitating blades are thereby completely submerged even further.
The fermentation substrates used for generating biogas usually have structurally viscous flow properties. Structurally viscous means that the dynamic viscosity of the fermentation substrate decreases with an increase in shear rate. Viscosity is therefore not a value, but a function. For each induced shear rate, an associated viscosity is obtained. The viscosity in the container is consequently locally different. It depends on the shear rates present locally. The reason for this is the local velocities which influence the flow in the container.
Shear rates are generated by the movement of the propeller of an agitator mechanism. In the surroundings of the propeller, the local viscosity decreases in the case of structurally viscous fermentation substrates. With an increase in distance from the propeller, the shear rate is reduced and the viscosity rises correspondingly. The result of this is that the propeller predominantly sucks in fermentation substrate from near-propeller regions where the fermentation substrate has a low viscosity. This gives rise to near-propeller regions in which the substrate is transported at high velocities in a small volume only around the propeller itself. These near-propeller regions are designated as caverns. Where agitator mechanisms operate only locally in a cavern, optimal intermixing of the container contents does not take place because the generation of flow is restricted to these regions. Consequently, this leads to a reduction in the useful reactor volume in relation to the actual capacity of the bioreactor. As a result, less biogas and therefore also less useful methane are generated in the smaller useful reactor volume. The methane fraction or methane quantity has effects upon the economically efficient operation of a bioreactor.